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Terra Preta Soils: The Benefits of Biochar

Thousands of years ago, the Amazon Indians created Terra Preta soils, which are, in every which way, the most amazingly productive form of agriculture. It is 300% more productive than the very best modern efforts – and this without any chemical fertilizers, equipment, and application costs.

Left – a nutrient-depleted soil; Right – a soil transformed into fertile Terra Preta

The indigenous farmers in these regions simply dug a deep ditch and, starting at one end, filled it in progressively with their household and personal wastes, covering it with a layer of soil as they went along, until the whole ditch had been filled in again and covered over with soil. Then they started another ditch alongside the first, and so forth, and so forth, until the whole plot was essentially a covered compost field. Then they started over again at the beginning.

In this manner, they created what has now become known as “Terra Preta” soil (“dark soil” in Portuguese), an incredibly rich, fertile, beautifully friable and supremely productive soil. Some plots are 8 feet deep and at least one plot is formally known to have produced abundant crops for 40 years without any fertilizers whatsoever.

The difference between this soil a simple ‘deep’ compost soil is the presence of charcoal and pottery shards in the Terra Preta soils. The charcoal comes from two sources: from burning forest plots to clear land for growing their crops, and from the remnants of cooking fires. Unlike tiny tidbits of ash, these coarse lumps of charcoal (referred to as biochar) are full of crevices and holes, which help them serve as life rafts to soil microorganisms. The carbon compounds in charcoal form loose chemical bonds with soluble plant nutrients so they are not as readily washed away by rain and irrigation. When used alone, Biochar has little benefit to plants, but when used in combination with compost and organic fertilizers, it can dramatically improve plant growth while helping retain nutrients in the soil.

Because nobody had ever taken the time to investigate powdered charcoal’s effects on soil fertility carefully, soil scientists had simply always assumed that charcoal was inert when added to the soil. However, what soil scientists, working with microbiologists, discovered was that an entire community of bacteria exists in symbiosis with the root hairs of plants in terra preta soils. The bacteria produce enzymes that release the mineral ions trapped in the charcoal and make it available to the root hairs of the plant as nutrients. In return, the plants secrete nourishment for the bacteria.

There is an ecology going on in these soils that is not completely understood, but if replicated it could have multiple benefits for farmers and environmentalists. Terra Preta creates a terrestrial carbon reef at a microscopic level. If we could succeed in recreating terra preta, it could spare the rainforest from destruction and help feed people across the developing world. This simple melding of soil and fire, first discovered by ancient people in the Amazon, may be the key to feeding ourselves while restoring the health of our planet.